1. Field of the Invention
The present invention relates to an improved heater for air data sensing devices and in particular to regulating the temperature of such air data sensors through the use of PTC resistive material.
2. Prior Art
As used herein, a conventional resistive heater is considered one which does not substantially increase in resistance across its range of operating temperatures. A PTC heater has the property of increasing in resistance a substantial amount at a temperature within its normal operating range.
In the prior art the use of PTC resistive material for various heating devices has been disclosed. For example, in U.S. Pat. No. 4,121,088, held by the same assignee as the present invention, a PTC resistive heater is used in combination with an angle of attack vane to provide automatic means for maintaining the temperature of the vane at a level which will de-ice the vane. The preferred means of connecting and insulating the PTC heater elements are disclosed therein and incorporated herein by reference. Though the device in U.S. Pat. No. 4,121,088 has received favorable commercial acceptance, it is distinct from the present invention in that the vane heater consists solely of the PTC resistive material.
In U.S. Pat. No. 3,488,470, an electrical heating element is embedded in an electrical insulating sheath in a pressure head for aircraft. Further disclosed therein is the element of wire of the type having a high temperature resistance coefficient for providing automatic thermal regulation.
In U.S. Pat. No. 4,000,647, a plurality of thermally controlled resistance means are annularly deposited in a probe (sonde). Two practical constraints in the design of a probe are, one, to minimize probe size in order to minimize aerodynamic drag therefrom, thereby maximizing the contribution of the probe to the fuel efficiency of the aircraft and, two, to provide adequate heating of the probe, especially in the tip area of the probe that is most critical with regard to icing. The result is that the heating means selected must be constructed in a very constricted space, especially in the critical tip area. Due to their relatively large size and their limitations on possible shape, known PTC resistive heaters have not been readily adaptable for use in such probes.
PTC material has also found wide acceptance as a switch capable of providing heat protection as exemplified by affixing the material to an electrical motor to switch the motor off if temperature exceeds a certain value.
U.S. Pat. No. 3,374,774, Fully Automatic Electric Coffee Pot, is an extension of this concept in that it uses the PTC material essentially to switch from one heater to another at a certain temperature. This invention has a heating unit for the liquid which consists of a conventional alloy resistive heater connected in series with a PTC resistive heater made of barium-titanate. In this application, the liquid is first heated to boiling by the conventional alloy resistive heater to brew the coffee and then, using the PTC characteristic, the conventional alloy resistive heater is substantially switched off and, simultaneously, the lower heat output PTC resistive heater is switched from being substantially off to on, to keep the coffee warm without further boiling. As described, the PTC characteristic is used to digitally switch from a high output heater to a low output heater when the desired conditions exist.
Present heating of flow sensor probes for air vehicles is done with resistive heaters, typically constructed of Ni-Cr (nickel-chromium) alloy. The maximum rated power of the heater must be such that sufficient heat for satisfactory performance of the sensor under the most severe icing conditions is supplied. In order to reduce aircrew task loading, such heaters are typically automatically energized when the aircraft is operating on internal power. The heater then operates continually at maximum rated power. The result in that while the aircraft is on the ground, which is the condition of minimum heat dissipation from the sensor, the heater causes inordinately high temperatures in the sensor. Temperatures of 550.degree. C. are not uncommon. Such temperatures may result in burn out of the heater and also contribute to erosion of the probe features due to impingement of salt in the airstream when airborne at low altitude, both of which affect the accuracy of the instrument. Such temperatures also contribute to the creation of a safety hazard. Accordingly, it is desirable to provide a heater power control system that will continuously vary heater power in response to rate of heat dissipation being experienced at the external surface of the sensor. In order to preserve the reliability of the sensor system, it is desirable that the heater power control system be passive as opposed to an active electronic system.